In the past few decades, the petroleum industry has invested heavily in the development of marine seismic survey techniques that yield knowledge of subterranean formations beneath a body of water in order to find and extract valuable mineral resources, such as oil. High-resolution seismic images of a subterranean formation are essential for quantitative seismic interpretation and reservoir monitoring. For a typical marine seismic survey, a survey vessel tows a source and the same or another survey vessel tows one or more streamers that form a seismic data acquisition surface below the surface of the water and over a subterranean formation to be surveyed for mineral deposits. The survey vessel contains seismic acquisition equipment, such as navigation control, source control, seismic receiver control, and recording equipment. The source control activates the source, which is typically an array of source elements, such as air guns, to produce acoustic impulses at selected times. Activation of the source is also described as being “fired” or “shot” to produce the acoustic impulse. Each acoustic impulse is a sound wave that travels down through the water and into the subterranean formation. At each interface between different types of rock, a portion of the sound wave is refracted and another portion is reflected back toward the body of water to propagate toward the surface. The streamers towed behind the survey vessel are elongated cable-like structures. Each streamer may include a number of seismic receivers or dual sensors that detect pressure and particle motion wavefields associated with the sound waves reflected back into the water from the subterranean formation.
In a typical seismic marine survey, the source is activated on position. When the survey vessel tows the source at a substantially constant rate of speed, the source is fired at substantially regular intervals of time and seismic data is recorded during waiting periods (i.e., typically 10 seconds) between shots. A small variation in waiting periods is typically in the range of 0.1-0.2 seconds. Although the shot time may vary within this range, the time at which the source is fired is recorded precisely. Thus primary seismic data can be aligned from shot to shot. However, acoustic energy associated with sound waves reflected from the subterranean formation does not stop emanating from the subterranean formation at the end of a waiting period between shots. The acoustic energy recorded in one waiting period lingers in one or more subsequent waiting periods and is referred to as “shot generated noise.” This noise is actually acoustic energy generated by the source, which is then reflected or scattered from anywhere in the subsurface. Although this energy may be low in amplitude within a single shot, in the lower frequency parts of the spectrum it is coherent, so it does add constructively in the various summation and imaging processes used to analyze seismic data. Obtaining high signal to noise levels at low frequencies is particularly important in seismic imaging and this shot generated coherent energy effectively acts as a noise source that contaminates the seismic images. As a result, those working in the petroleum industry seek systems and methods to reduce the effects of shot generated noise.